Import macros from gnulib (often changing their name).

Import macros from gnulib (often changing their name).

* NEWS: AC_C_TYPE_LONG_DOUBLE is now obsolete.
New macros AC_CHECK_DECLS_ONCE, AC_CHECK_FUNCS_ONCE,
AC_CHECK_HEADERS_ONCE, AC_FUNC_STRTOLD, AC_HEADER_ASSERT,
AC_STRUCT_DIRENT_D_INO, AC_STRUCT_DIRENT_D_TYPE,
AC_TYPE_LONG_DOUBLE, AC_TYPE_LONG_DOUBLE_WIDER, AC_TYPE_INT8_T,
AC_TYPE_INT16_T, AC_TYPE_INT32_T, AC_TYPE_INT64_T,
AC_TYPE_INTMAX_T, AC_TYPE_INTPTR_T, AC_TYPE_LONG_LONG_INT,
AC_TYPE_UINT8_T, AC_TYPE_UINT16_T, AC_TYPE_UINT32_T,
AC_TYPE_UINT64_T, AC_TYPE_UINTMAX_T, AC_TYPE_UINTPTR_T,
AC_TYPE_UNSIGNED_LONG_LONG_INT, AC_USE_SYSTEM_EXTENSIONS.
The manual mentions Gnulib more prominently.
* doc/autoconf.texi (Gnulib): New node.
(Pointers): Add Gnulib URL.
(Particular Functions): Alphabetize.  Add AC_FUNC_STRTOLD.
(Generic Functions): Add AC_CHECK_FUNCS_ONCE.  Refer to new
Gnulib section.
(Particular Headers): Add AC_HEADER_ASSERT.  For stdbool.h,
suggest a #define rather than a typedef for _Bool, and mention
Gnulib rather than trying to substitute stdbool code.
(Generic Headers): Add AC_CHECK_HEADERS_ONCE.
(Generic Declarations): Add AC_CHECK_DECLS_ONCE.
(Particular Structures): Add AC_STRUCT_DIRENT_D_INO,
AC_STRUCT_DIRENT_D_TYPE.
(Particular Types): Mention stdint.h and inttypes.h as standard
headers too.
Add AC_TYPE_INT8_T, AC_TYPE_INT16_T, AC_TYPE_INT32_T, AC_TYPE_INT64_T,
AC_TYPE_INTMAX_T, AC_TYPE_INTPTR_T, AC_TYPE_LONG_DOUBLE,
AC_TYPE_LONG_DOUBLE_WIDER, AC_TYPE_LONG_LONG_INT, AC_TYPE_UINT8_T,
AC_TYPE_UINT16_T, AC_TYPE_UINT32_T, AC_TYPE_UINT64_T, AC_TYPE_UINTMAX_T,
AC_TYPE_UINTPTR_T, AC_TYPE_UNSIGNED_LONG_LONG_INT.
(C Compiler): Move AC_C_LONG_DOUBLE to ...
(Obsolete Macros): here.  Under AC_LONG_DOUBLE, mention
AC_TYPE_LONG_DOUBLE or AC_TYPE_LONG_DOUBLE_WIDER instead.
(Posix Variants): Add AC_USE_SYSTEM_EXTENSIONS.
(Coding Style).  Don't mention m4_expand_once.
* lib/autoconf/c.m4 (AC_C_LONG_DOUBLE): Implement via
AC_TYPE_LONG_DOUBLE_WIDER.  Now obsolete.
* lib/autoconf/functions.m4 (_AH_CHECK_FUNCS): New macro.
(AC_CHECK_FUNCS): Use it.
(AC_CHECK_FUNCS_ONCE, AC_FUNC_STRTOLD): New macros.
(AC_FUNC_WAIT3): "the Open Group standards" -> "POSIX".
* lib/autoconf/general.m4 (AC_CHECK_DECLS_ONCE): New macro.
* lib/autoconf/headers.m4 (AC_CHECK_HEADERS_ONCE): New macro.
(AC_HEADER_ASSERT): New macro.
(AC_HEADER_STDBOOL): Don't assume "#error" works.
Catch a bug in IBM AIX xlc compiler version 6.0.0.0.
Catch a bug in an HP-UX C compiler.
* lib/autoconf/specific.m4 (AC_USE_SYSTEM_EXTENSIONS): New macro.
* lib/autoconf/types.m4 (AC_TYPE_INTMAX_T. AC_TYPE_UINTMAX_T):
(AC_TYPE_INTPTR_T, AC_TYPE_UINTPTR_T. AC_TYPE_LONG_DOUBLE):
(AC_TYPE_LONG_DOUBLE_WIDER, AC_C_LONG_DOUBLE, AC_TYPE_LONG_LONG_INT):
(AC_TYPE_UNSIGNED_LONG_LONG_INT, _AC_TYPE_INT, _AC_TYPE_UNSIGNED_INT):
(_AC_STRUCT_DIRENT, AC_STRUCT_DIRENT_D_INO, AC_STRUCT_DIRENT_D_TYPE):
New macros.